Big Bang theory is the prevailing cosmological model for the
universe from the earliest known periods through its subsequent
large-scale evolution. The model describes how the universe
expanded from a very high-density and high-temperature state,
and offers a comprehensive explanation for a broad range of phenomena,
including the abundance of light elements, the cosmic microwave
background (CMB), large scale structure and Hubble's law. If the
known laws of physics are extrapolated to the highest density regime,
the result is a singularity which is typically associated with the Big
Bang. Physicists are undecided whether this means the universe began
from a singularity, or that current knowledge is insufficient to
describe the universe at that time. Detailed measurements of the
expansion rate of the universe place the
Big Bang at around 13.8
billion years ago, which is thus considered the age of the
universe. After the initial expansion, the universe cooled
sufficiently to allow the formation of subatomic particles, and later
simple atoms. Giant clouds of these primordial elements later
coalesced through gravity in halos of dark matter, eventually forming
the stars and galaxies visible today.
Georges Lemaître first noted in 1927 that an expanding universe
could be traced back in time to an originating single point,
scientists have built on his idea of cosmic expansion. The scientific
community was once divided between supporters of two different
Big Bang and the Steady State theory, but a wide range
of empirical evidence has strongly favored the
Big Bang which is now
universally accepted. In 1929, from analysis of galactic redshifts,
Edwin Hubble concluded that galaxies are drifting apart; this is
important observational evidence consistent with the hypothesis of an
expanding universe. In 1964, the cosmic microwave background radiation
was discovered, which was crucial evidence in favor of the Big Bang
model, since that theory predicted the existence of background
radiation throughout the universe before it was discovered. More
recently, measurements of the redshifts of supernovae indicate that
the expansion of the universe is accelerating, an observation
attributed to dark energy's existence. The known physical laws of
nature can be used to calculate the characteristics of the universe in
detail back in time to an initial state of extreme density and
2.2 Inflation and baryogenesis
2.4 Structure formation
2.5 Cosmic acceleration
3 Features of the model
3.1 Expansion of space
5 Observational evidence
Hubble's law and the expansion of space
Cosmic microwave background
Cosmic microwave background radiation
5.3 Abundance of primordial elements
5.4 Galactic evolution and distribution
5.5 Primordial gas clouds
5.6 Other lines of evidence
5.7 Future observations
6 Problems and related issues in physics
6.2 Dark energy
6.3 Dark matter
6.4 Horizon problem
6.5 Magnetic monopoles
6.6 Flatness problem
8 Ultimate fate of the universe
11 Religious and philosophical interpretations
12 See also
15 Further reading
16 External links
view • discuss • edit
Earliest universe (−13.80)
Omega Centauri forms
Milky Way Galaxy
spiral arms form
Alpha Centauri forms
Earliest Earth (−4.54)
Earliest sexual reproduction
Axis scale: billion years
Human timeline and
A graphical timeline is available at
Graphical timeline of the Big Bang
Edwin Hubble observed that the distances to
faraway galaxies were strongly correlated with their redshifts. This
was interpreted to mean that all distant galaxies and clusters are
receding away from our vantage point with an apparent velocity
proportional to their distance: that is, the farther they are, the
faster they move away from us, regardless of direction. Assuming
Copernican principle (that the Earth is not the center of the
universe), the only remaining interpretation is that all observable
regions of the universe are receding from all others. Since we know
that the distance between galaxies increases today, it must mean that
in the past galaxies were closer together. The continuous expansion of
the universe implies that the universe was denser and hotter in the
Large particle accelerators can replicate the conditions that
prevailed after the early moments of the universe, resulting in
confirmation and refinement of the details of the
Big Bang model.
However, these accelerators can only probe so far into high energy
regimes. Consequently, the state of the universe in the earliest
instants of the
Big Bang expansion is still poorly understood and an
area of open investigation and speculation.
The first subatomic particles to be formed included protons, neutrons,
and electrons. Though simple atomic nuclei formed within the first
three minutes after the Big Bang, thousands of years passed before the
first electrically neutral atoms formed. The majority of atoms
produced by the
Big Bang were hydrogen, along with helium and traces
of lithium. Giant clouds of these primordial elements later coalesced
through gravity to form stars and galaxies, and the heavier elements
were synthesized either within stars or during supernovae.
Big Bang theory offers a comprehensive explanation for a broad
range of observed phenomena, including the abundance of light
elements, the CMB, large scale structure, and Hubble's Law. The
framework for the
Big Bang model relies on Albert Einstein's theory of
general relativity and on simplifying assumptions such as homogeneity
and isotropy of space. The governing equations were formulated by
Alexander Friedmann, and similar solutions were worked on by Willem de
Sitter. Since then, astrophysicists have incorporated observational
and theoretical additions into the
Big Bang model, and its
parametrization as the
Lambda-CDM model serves as the framework for
current investigations of theoretical cosmology. The Lambda-CDM model
is the current "standard model" of
Big Bang cosmology, consensus is
that it is the simplest model that can account for the various
measurements and observations relevant to cosmology.
Main article: Chronology of the universe
Gravitational singularity and Planck epoch
Extrapolation of the expansion of the universe backwards in time using
general relativity yields an infinite density and temperature at a
finite time in the past. This singularity indicates that general
relativity is not an adequate description of the laws of physics in
this regime. Models based on general relativity alone can not
extrapolate toward the singularity beyond the end of the Planck epoch.
This primordial singularity is itself sometimes called "the Big
Bang", but the term can also refer to a more generic early hot,
dense phase[notes 1] of the universe. In either case, "the Big
Bang" as an event is also colloquially referred to as the "birth" of
our universe since it represents the point in history where the
universe can be verified to have entered into a regime where the laws
of physics as we understand them (specifically general relativity and
the standard model of particle physics) work. Based on measurements of
the expansion using Type Ia supernovae and measurements of temperature
fluctuations in the cosmic microwave background, the time that has
passed since that event — otherwise known as the "age of the
universe" — is 13.799 ± 0.021 billion years. The agreement
of independent measurements of this age supports the ΛCDM model that
describes in detail the characteristics of the universe.
Despite being extremely dense at this time—far denser than is
usually required to form a black hole—the universe did not
re-collapse into a black hole. This may be explained by considering
that commonly-used calculations and limits for gravitational collapse
are usually based upon objects of relatively constant size, such as
stars, and do not apply to rapidly expanding space such as the Big
Inflation and baryogenesis
Cosmic inflation and baryogenesis
The earliest phases of the
Big Bang are subject to much speculation.
In the most common models the universe was filled homogeneously and
isotropically with a very high energy density and huge temperatures
and pressures and was very rapidly expanding and cooling.
Approximately 10−37 seconds into the expansion, a phase transition
caused a cosmic inflation, during which the universe grew
exponentially during which time density fluctuations that occurred
because of the uncertainty principle were amplified into the seeds
that would later form the large-scale structure of the universe.
After inflation stopped, reheating occurred until the universe
obtained the temperatures required for the production of a
quark–gluon plasma as well as all other elementary particles.
Temperatures were so high that the random motions of particles were at
relativistic speeds, and particle–antiparticle pairs of all kinds
were being continuously created and destroyed in collisions. At
some point, an unknown reaction called baryogenesis violated the
conservation of baryon number, leading to a very small excess of
quarks and leptons over antiquarks and antileptons—of the order of
one part in 30 million. This resulted in the predominance of
matter over antimatter in the present universe.
Big Bang nucleosynthesis
Big Bang nucleosynthesis and cosmic microwave
Panoramic view of the entire near-infrared sky reveals the
distribution of galaxies beyond the Milky Way.
color-coded by redshift.
The universe continued to decrease in density and fall in temperature,
hence the typical energy of each particle was decreasing. Symmetry
breaking phase transitions put the fundamental forces of physics and
the parameters of elementary particles into their present form.
After about 10−11 seconds, the picture becomes less speculative,
since particle energies drop to values that can be attained in
particle accelerators. At about 10−6 seconds, quarks and gluons
combined to form baryons such as protons and neutrons. The small
excess of quarks over antiquarks led to a small excess of baryons over
antibaryons. The temperature was now no longer high enough to create
new proton–antiproton pairs (similarly for neutrons–antineutrons),
so a mass annihilation immediately followed, leaving just one in 1010
of the original protons and neutrons, and none of their antiparticles.
A similar process happened at about 1 second for electrons and
positrons. After these annihilations, the remaining protons, neutrons
and electrons were no longer moving relativistically and the energy
density of the universe was dominated by photons (with a minor
contribution from neutrinos).
A few minutes into the expansion, when the temperature was about a
billion (one thousand million) kelvin and the density was about that
of air, neutrons combined with protons to form the universe's
deuterium and helium nuclei in a process called Big Bang
nucleosynthesis. Most protons remained uncombined as hydrogen
As the universe cooled, the rest mass energy density of matter came to
gravitationally dominate that of the photon radiation. After about
379,000 years, the electrons and nuclei combined into atoms (mostly
hydrogen); hence the radiation decoupled from matter and continued
through space largely unimpeded. This relic radiation is known as the
cosmic microwave background radiation. The chemistry of life may
have begun shortly after the Big Bang, 13.8 billion years ago, during
a habitable epoch when the universe was only 10–17 million years
Main article: Structure formation
Abell 2744 galaxy cluster - Hubble Frontier Fields view.
Over a long period of time, the slightly denser regions of the nearly
uniformly distributed matter gravitationally attracted nearby matter
and thus grew even denser, forming gas clouds, stars, galaxies, and
the other astronomical structures observable today. The details of
this process depend on the amount and type of matter in the universe.
The four possible types of matter are known as cold dark matter, warm
dark matter, hot dark matter, and baryonic matter. The best
measurements available, from Wilkinson
Microwave Anisotropy Probe
(WMAP), show that the data is well-fit by a
Lambda-CDM model in which
dark matter is assumed to be cold (warm dark matter is ruled out by
early reionization), and is estimated to make up about 23% of the
matter/energy of the universe, while baryonic matter makes up about
4.6%. In an "extended model" which includes hot dark matter in the
form of neutrinos, then if the "physical baryon density"
displaystyle Omega _ text b h^ 2
is estimated at about 0.023 (this is different from the 'baryon
displaystyle Omega _ text b
expressed as a fraction of the total matter/energy density, which as
noted above is about 0.046), and the corresponding cold dark matter
density Ωch2 is about 0.11, the corresponding neutrino density Ωvh2
is estimated to be less than 0.0062.
Main article: Accelerating expansion of the universe
Independent lines of evidence from Type Ia supernovae and the CMB
imply that the universe today is dominated by a mysterious form of
energy known as dark energy, which apparently permeates all of space.
The observations suggest 73% of the total energy density of today's
universe is in this form. When the universe was very young, it was
likely infused with dark energy, but with less space and everything
closer together, gravity predominated, and it was slowly braking the
expansion. But eventually, after numerous billion years of expansion,
the growing abundance of dark energy caused the expansion of the
universe to slowly begin to accelerate.
Dark energy in its simplest formulation takes the form of the
cosmological constant term in Einstein's field equations of general
relativity, but its composition and mechanism are unknown and, more
generally, the details of its equation of state and relationship with
Standard Model of particle physics continue to be investigated
both through observation and theoretically.
All of this cosmic evolution after the inflationary epoch can be
rigorously described and modeled by the ΛCDM model of cosmology,
which uses the independent frameworks of quantum mechanics and
Einstein's General Relativity. There is no well-supported model
describing the action prior to 10−15 seconds or so. Apparently a new
unified theory of quantum gravitation is needed to break this barrier.
Understanding this earliest of eras in the history of the universe is
currently one of the greatest unsolved problems in physics.
Features of the model
Big Bang theory depends on two major assumptions: the universality
of physical laws and the cosmological principle. The cosmological
principle states that on large scales the universe is homogeneous and
These ideas were initially taken as postulates, but today there are
efforts to test each of them. For example, the first assumption has
been tested by observations showing that largest possible deviation of
the fine structure constant over much of the age of the universe is of
order 10−5. Also, general relativity has passed stringent tests
on the scale of the Solar System and binary stars.[notes 2]
If the large-scale universe appears isotropic as viewed from Earth,
the cosmological principle can be derived from the simpler Copernican
principle, which states that there is no preferred (or special)
observer or vantage point. To this end, the cosmological principle has
been confirmed to a level of 10−5 via observations of the CMB. The
universe has been measured to be homogeneous on the largest scales at
the 10% level.
Expansion of space
Friedmann–Lemaître–Robertson–Walker metric and
Metric expansion of space
General relativity describes spacetime by a metric, which determines
the distances that separate nearby points. The points, which can be
galaxies, stars, or other objects, are themselves specified using a
coordinate chart or "grid" that is laid down over all spacetime. The
cosmological principle implies that the metric should be homogeneous
and isotropic on large scales, which uniquely singles out the
Friedmann–Lemaître–Robertson–Walker metric (FLRW metric). This
metric contains a scale factor, which describes how the size of the
universe changes with time. This enables a convenient choice of a
coordinate system to be made, called comoving coordinates. In this
coordinate system, the grid expands along with the universe, and
objects that are moving only because of the expansion of the universe,
remain at fixed points on the grid. While their coordinate distance
(comoving distance) remains constant, the physical distance between
two such co-moving points expands proportionally with the scale factor
of the universe.
Big Bang is not an explosion of matter moving outward to fill an
empty universe. Instead, space itself expands with time everywhere and
increases the physical distance between two comoving points. In other
Big Bang is not an explosion in space, but rather an
expansion of space. Because the FLRW metric assumes a uniform
distribution of mass and energy, it applies to our universe only on
large scales—local concentrations of matter such as our galaxy are
gravitationally bound and as such do not experience the large-scale
expansion of space.
Main article: List of cosmological horizons
An important feature of the
Big Bang spacetime is the presence of
particle horizons. Since the universe has a finite age, and light
travels at a finite speed, there may be events in the past whose light
has not had time to reach us. This places a limit or a past horizon on
the most distant objects that can be observed. Conversely, because
space is expanding, and more distant objects are receding ever more
quickly, light emitted by us today may never "catch up" to very
distant objects. This defines a future horizon, which limits the
events in the future that we will be able to influence. The presence
of either type of horizon depends on the details of the FLRW model
that describes our universe.
Our understanding of the universe back to very early times suggests
that there is a past horizon, though in practice our view is also
limited by the opacity of the universe at early times. So our view
cannot extend further backward in time, though the horizon recedes in
space. If the expansion of the universe continues to accelerate, there
is a future horizon as well.
Main article: History of the
Big Bang theory
See also: Timeline of cosmological theories
Fred Hoyle is credited with coining the term "Big
Bang" during a 1949 BBC radio broadcast, saying: "These theories were
based on the hypothesis that all the matter in the universe was
created in one big bang at a particular time in the remote past."
It is popularly reported that Hoyle, who favored an alternative
"steady state" cosmological model, intended this to be pejorative,
but Hoyle explicitly denied this and said it was just a striking image
meant to highlight the difference between the two
Hubble eXtreme Deep Field (XDF)
XDF size compared to the size of the
Moon - several thousand galaxies,
each consisting of billions of stars, are in this small view.
XDF (2012) view - each light speck is a galaxy - some of these are as
old as 13.2 billion years - the universe is estimated to contain
200 billion galaxies.
XDF image shows fully mature galaxies in the foreground plane - nearly
mature galaxies from 5 to 9 billion years ago - protogalaxies, blazing
with young stars, beyond 9 billion years.
Big Bang theory developed from observations of the structure of
the universe and from theoretical considerations. In 1912 Vesto
Slipher measured the first
Doppler shift of a "spiral nebula" (spiral
nebula is the obsolete term for spiral galaxies), and soon discovered
that almost all such nebulae were receding from Earth. He did not
grasp the cosmological implications of this fact, and indeed at the
time it was highly controversial whether or not these nebulae were
"island universes" outside our Milky Way. Ten years later,
Alexander Friedmann, a Russian cosmologist and mathematician, derived
Friedmann equations from Albert Einstein's equations of general
relativity, showing that the universe might be expanding in contrast
to the static universe model advocated by Einstein at that time.
In 1924 Edwin Hubble's measurement of the great distance to the
nearest spiral nebulae showed that these systems were indeed other
galaxies. Independently deriving Friedmann's equations in 1927,
Georges Lemaître, a Belgian physicist and
Roman Catholic priest,
proposed that the inferred recession of the nebulae was due to the
expansion of the universe.
In 1931 Lemaître went further and suggested that the evident
expansion of the universe, if projected back in time, meant that the
further in the past the smaller the universe was, until at some finite
time in the past all the mass of the universe was concentrated into a
single point, a "primeval atom" where and when the fabric of time and
space came into existence.
Starting in 1924, Hubble painstakingly developed a series of distance
indicators, the forerunner of the cosmic distance ladder, using the
100-inch (2.5 m)
Hooker telescope at Mount Wilson Observatory.
This allowed him to estimate distances to galaxies whose redshifts had
already been measured, mostly by Slipher. In 1929 Hubble discovered a
correlation between distance and recession velocity—now known as
Hubble's law. Lemaître had already shown that this was
expected, given the cosmological principle.
In the 1920s and 1930s almost every major cosmologist preferred an
eternal steady state universe, and several complained that the
beginning of time implied by the
Big Bang imported religious concepts
into physics; this objection was later repeated by supporters of the
steady state theory. This perception was enhanced by the fact that
the originator of the
Big Bang theory, Georges Lemaître, was a Roman
Arthur Eddington agreed with
Aristotle that the
universe did not have a beginning in time, viz., that matter is
eternal. A beginning in time was "repugnant" to him.
Lemaître, however, thought that
If the world has begun with a single quantum, the notions of space and
time would altogether fail to have any meaning at the beginning; they
would only begin to have a sensible meaning when the original quantum
had been divided into a sufficient number of quanta. If this
suggestion is correct, the beginning of the world happened a little
before the beginning of space and time.
During the 1930s other ideas were proposed as non-standard cosmologies
to explain Hubble's observations, including the Milne model, the
oscillatory universe (originally suggested by Friedmann, but advocated
Albert Einstein and Richard Tolman) and Fritz Zwicky's tired
After World War II, two distinct possibilities emerged. One was Fred
Hoyle's steady state model, whereby new matter would be created as the
universe seemed to expand. In this model the universe is roughly the
same at any point in time. The other was Lemaître's Big Bang
theory, advocated and developed by George Gamow, who introduced big
bang nucleosynthesis (BBN) and whose associates,
Ralph Alpher and
Robert Herman, predicted the CMB. Ironically, it was Hoyle who
coined the phrase that came to be applied to Lemaître's theory,
referring to it as "this big bang idea" during a
BBC Radio broadcast
in March 1949.[notes 3] For a while, support was split between
these two theories. Eventually, the observational evidence, most
notably from radio source counts, began to favor
Big Bang over Steady
State. The discovery and confirmation of the CMB in 1964 secured the
Big Bang as the best theory of the origin and evolution of the
universe. Much of the current work in cosmology includes
understanding how galaxies form in the context of the Big Bang,
understanding the physics of the universe at earlier and earlier
times, and reconciling observations with the basic theory.
In 1968 and 1970 Roger Penrose, Stephen Hawking, and George F. R.
Ellis published papers where they showed that mathematical
singularities were an inevitable initial condition of general
relativistic models of the Big Bang. Then, from the 1970s to
the 1990s, cosmologists worked on characterizing the features of the
Big Bang universe and resolving outstanding problems. In 1981, Alan
Guth made a breakthrough in theoretical work on resolving certain
outstanding theoretical problems in the
Big Bang theory with the
introduction of an epoch of rapid expansion in the early universe he
called "inflation". Meanwhile, during these decades, two questions
in observational cosmology that generated much discussion and
disagreement were over the precise values of the Hubble Constant
and the matter-density of the universe (before the discovery of dark
energy, thought to be the key predictor for the eventual fate of the
In the mid-1990s, observations of certain globular clusters appeared
to indicate, that they were about 15 billion years old, which
conflicted with most then-current estimates of the age of the universe
(and indeed with the age measured today). This issue was later
resolved when new computer simulations, which included the effects of
mass loss due to stellar winds, indicated a much younger age for
globular clusters. While there still remain some questions as to
how accurately the ages of the clusters are measured, globular
clusters are of interest to cosmology as some of the oldest objects in
Significant progress in
Big Bang cosmology has been made since the
late 1990s as a result of advances in telescope technology as well as
the analysis of data from satellites such as COBE, the Hubble
Telescope and WMAP. Cosmologists now have fairly precise and
accurate measurements of many of the parameters of the
Big Bang model,
and have made the unexpected discovery that the expansion of the
universe appears to be accelerating.
Artist's depiction of the
WMAP satellite gathering data to help
scientists understand the Big Bang
"[The] big bang picture is too firmly grounded in data from every area
to be proved invalid in its general features."
The earliest and most direct observational evidence of the validity of
the theory are the expansion of the universe according to Hubble's law
(as indicated by the redshifts of galaxies), discovery and measurement
of the cosmic microwave background and the relative abundances of
light elements produced by
Big Bang nucleosynthesis. More recent
evidence includes observations of galaxy formation and evolution, and
the distribution of large-scale cosmic structures, These are
sometimes called the "four pillars" of the
Big Bang theory.
Precise modern models of the
Big Bang appeal to various exotic
physical phenomena that have not been observed in terrestrial
laboratory experiments or incorporated into the
Standard Model of
particle physics. Of these features, dark matter is currently
subjected to the most active laboratory investigations. Remaining
issues include the cuspy halo problem and the dwarf galaxy problem of
cold dark matter.
Dark energy is also an area of intense interest for
scientists, but it is not clear whether direct detection of dark
energy will be possible. Inflation and baryogenesis remain more
speculative features of current
Big Bang models. Viable, quantitative
explanations for such phenomena are still being sought. These are
currently unsolved problems in physics.
Hubble's law and the expansion of space
Hubble's law and Metric expansion of space
Distance measures (cosmology)
Distance measures (cosmology) and
Scale factor (universe)
Observations of distant galaxies and quasars show that these objects
are redshifted—the light emitted from them has been shifted to
longer wavelengths. This can be seen by taking a frequency spectrum of
an object and matching the spectroscopic pattern of emission lines or
absorption lines corresponding to atoms of the chemical elements
interacting with the light. These redshifts are uniformly isotropic,
distributed evenly among the observed objects in all directions. If
the redshift is interpreted as a Doppler shift, the recessional
velocity of the object can be calculated. For some galaxies, it is
possible to estimate distances via the cosmic distance ladder. When
the recessional velocities are plotted against these distances, a
linear relationship known as
Hubble's law is observed:
displaystyle v=H_ 0 D
is the recessional velocity of the galaxy or other distant object,
is the comoving distance to the object, and
displaystyle H_ 0
is Hubble's constant, measured to be 7001704000000000000♠70.4+1.3
−1.4 km/s/Mpc by the
Hubble's law has two possible explanations. Either we are at the
center of an explosion of galaxies—which is untenable given the
Copernican principle—or the universe is uniformly expanding
everywhere. This universal expansion was predicted from general
Alexander Friedmann in 1922 and
Georges Lemaître in
1927, well before Hubble made his 1929 analysis and observations,
and it remains the cornerstone of the
Big Bang theory as developed by
Friedmann, Lemaître, Robertson, and Walker.
The theory requires the relation v = HD to hold at all times, where D
is the comoving distance, v is the recessional velocity, and v, H, and
D vary as the universe expands (hence we write H0 to denote the
present-day Hubble "constant"). For distances much smaller than the
size of the observable universe, the Hubble redshift can be thought of
Doppler shift corresponding to the recession velocity v.
However, the redshift is not a true Doppler shift, but rather the
result of the expansion of the universe between the time the light was
emitted and the time that it was detected.
That space is undergoing metric expansion is shown by direct
observational evidence of the
Cosmological principle and the
Copernican principle, which together with
Hubble's law have no other
explanation. Astronomical redshifts are extremely isotropic and
homogeneous, supporting the
Cosmological principle that the
universe looks the same in all directions, along with much other
evidence. If the redshifts were the result of an explosion from a
center distant from us, they would not be so similar in different
Measurements of the effects of the cosmic microwave background
radiation on the dynamics of distant astrophysical systems in 2000
proved the Copernican principle, that, on a cosmological scale, the
Earth is not in a central position.
Radiation from the Big Bang
was demonstrably warmer at earlier times throughout the universe.
Uniform cooling of the CMB over billions of years is explainable only
if the universe is experiencing a metric expansion, and excludes the
possibility that we are near the unique center of an explosion.
Cosmic microwave background
Cosmic microwave background radiation
Cosmic microwave background
Cosmic microwave background radiation
WMAP image of the cosmic microwave background radiation
(2012). The radiation is isotropic to roughly one part in
Arno Penzias and Robert Wilson serendipitously discovered the
cosmic background radiation, an omnidirectional signal in the
microwave band. Their discovery provided substantial confirmation
of the big-bang predictions by Alpher, Herman and Gamow around 1950.
Through the 1970s the radiation was found to be approximately
consistent with a black body spectrum in all directions; this spectrum
has been redshifted by the expansion of the universe, and today
corresponds to approximately 2.725 K. This tipped the balance of
evidence in favor of the
Big Bang model, and Penzias and Wilson were
Nobel Prize in 1978.
The cosmic microwave background spectrum measured by the FIRAS
instrument on the COBE satellite is the most-precisely measured black
body spectrum in nature. The data points and error bars on this
graph are obscured by the theoretical curve.
The surface of last scattering corresponding to emission of the CMB
occurs shortly after recombination, the epoch when neutral hydrogen
becomes stable. Prior to this, the universe comprised a hot dense
photon-baryon plasma sea where photons were quickly scattered from
free charged particles. Peaking at around
7013117394272000000♠372±14 kyr, the mean free path for a
photon becomes long enough to reach the present day and the universe
NASA launched the
Cosmic Background Explorer
Cosmic Background Explorer satellite
(COBE), which made two major advances: in 1990, high-precision
spectrum measurements showed that the CMB frequency spectrum is an
almost perfect blackbody with no deviations at a level of 1 part in
104, and measured a residual temperature of 2.726 K (more recent
measurements have revised this figure down slightly to 2.7255 K);
then in 1992, further COBE measurements discovered tiny fluctuations
(anisotropies) in the CMB temperature across the sky, at a level of
about one part in 105.
John C. Mather
John C. Mather and
George Smoot were
awarded the 2006
Nobel Prize in Physics for their leadership in these
During the following decade, CMB anisotropies were further
investigated by a large number of ground-based and balloon
experiments. In 2000–2001 several experiments, most notably
BOOMERanG, found the shape of the universe to be spatially almost flat
by measuring the typical angular size (the size on the sky) of the
In early 2003, the first results of the Wilkinson
Probe (WMAP) were released, yielding what were at the time the most
accurate values for some of the cosmological parameters. The results
disproved several specific cosmic inflation models, but are consistent
with the inflation theory in general. The Planck space probe was
launched in May 2009. Other ground and balloon based cosmic microwave
background experiments are ongoing.
Abundance of primordial elements
Big Bang nucleosynthesis
Big Bang model it is possible to calculate the concentration
of helium-4, helium-3, deuterium, and lithium-7 in the universe as
ratios to the amount of ordinary hydrogen. The relative abundances
depend on a single parameter, the ratio of photons to baryons. This
value can be calculated independently from the detailed structure of
CMB fluctuations. The ratios predicted (by mass, not by number) are
about 0.25 for 4He/H, about 10−3 for 2H/H, about 10−4 for 3He/H
and about 10−9 for 7Li/H.
The measured abundances all agree at least roughly with those
predicted from a single value of the baryon-to-photon ratio. The
agreement is excellent for deuterium, close but formally discrepant
for 4He, and off by a factor of two for 7Li; in the latter two cases
there are substantial systematic uncertainties. Nonetheless, the
general consistency with abundances predicted by Big Bang
nucleosynthesis is strong evidence for the Big Bang, as the theory is
the only known explanation for the relative abundances of light
elements, and it is virtually impossible to "tune" the
Big Bang to
produce much more or less than 20–30% helium. Indeed, there is
no obvious reason outside of the
Big Bang that, for example, the young
universe (i.e., before star formation, as determined by studying
matter supposedly free of stellar nucleosynthesis products) should
have more helium than deuterium or more deuterium than 3He, and in
constant ratios, too.:182–185
Galactic evolution and distribution
Galaxy formation and evolution
Galaxy formation and evolution and Structure formation
Detailed observations of the morphology and distribution of galaxies
and quasars are in agreement with the current state of the Big Bang
theory. A combination of observations and theory suggest that the
first quasars and galaxies formed about a billion years after the Big
Bang, and since then, larger structures have been forming, such as
galaxy clusters and superclusters.
Populations of stars have been aging and evolving, so that distant
galaxies (which are observed as they were in the early universe)
appear very different from nearby galaxies (observed in a more recent
state). Moreover, galaxies that formed relatively recently, appear
markedly different from galaxies formed at similar distances but
shortly after the Big Bang. These observations are strong arguments
against the steady-state model. Observations of star formation, galaxy
and quasar distributions and larger structures, agree well with Big
Bang simulations of the formation of structure in the universe, and
are helping to complete details of the theory.
Primordial gas clouds
Focal plane of
BICEP2 telescope under a microscope - used to search
for polarization in the CMB.
In 2011, astronomers found what they believe to be pristine clouds of
primordial gas by analyzing absorption lines in the spectra of distant
quasars. Before this discovery, all other astronomical objects have
been observed to contain heavy elements that are formed in stars.
These two clouds of gas contain no elements heavier than hydrogen and
deuterium. Since the clouds of gas have no heavy elements,
they likely formed in the first few minutes after the Big Bang, during
Big Bang nucleosynthesis.
Other lines of evidence
The age of the universe as estimated from the Hubble expansion and the
CMB is now in good agreement with other estimates using the ages of
the oldest stars, both as measured by applying the theory of stellar
evolution to globular clusters and through radiometric dating of
Population II stars.
The prediction that the CMB temperature was higher in the past has
been experimentally supported by observations of very low temperature
absorption lines in gas clouds at high redshift. This prediction
also implies that the amplitude of the
Sunyaev–Zel'dovich effect in
clusters of galaxies does not depend directly on redshift.
Observations have found this to be roughly true, but this effect
depends on cluster properties that do change with cosmic time, making
precise measurements difficult.
Future gravitational waves observatories might be able to detect
primordial gravitational waves, relics of the early universe, up to
less than a second after the Big Bang.
Problems and related issues in physics
See also: List of unsolved problems in physics
As with any theory, a number of mysteries and problems have arisen as
a result of the development of the
Big Bang theory. Some of these
mysteries and problems have been resolved while others are still
outstanding. Proposed solutions to some of the problems in the Big
Bang model have revealed new mysteries of their own. For example, the
horizon problem, the magnetic monopole problem, and the flatness
problem are most commonly resolved with inflationary theory, but the
details of the inflationary universe are still left unresolved and
many, including some founders of the theory, say it has been
disproven. What follows are a list of the mysterious
aspects of the
Big Bang theory still under intense investigation by
cosmologists and astrophysicists.
It is not yet understood why the universe has more matter than
antimatter. It is generally assumed that when the universe was
young and very hot it was in statistical equilibrium and contained
equal numbers of baryons and antibaryons. However, observations
suggest that the universe, including its most distant parts, is made
almost entirely of matter. A process called baryogenesis was
hypothesized to account for the asymmetry. For baryogenesis to occur,
Sakharov conditions must be satisfied. These require that baryon
number is not conserved, that
CP-symmetry are violated
and that the universe depart from thermodynamic equilibrium. All
these conditions occur in the Standard Model, but the effects are not
strong enough to explain the present baryon asymmetry.
Main article: Dark energy
Measurements of the redshift–magnitude relation for type Ia
supernovae indicate that the expansion of the universe has been
accelerating since the universe was about half its present age. To
explain this acceleration, general relativity requires that much of
the energy in the universe consists of a component with large negative
pressure, dubbed "dark energy".
Dark energy, though speculative, solves numerous problems.
Measurements of the cosmic microwave background indicate that the
universe is very nearly spatially flat, and therefore according to
general relativity the universe must have almost exactly the critical
density of mass/energy. But the mass density of the universe can be
measured from its gravitational clustering, and is found to have only
about 30% of the critical density. Since theory suggests that dark
energy does not cluster in the usual way it is the best explanation
for the "missing" energy density.
Dark energy also helps to explain
two geometrical measures of the overall curvature of the universe, one
using the frequency of gravitational lenses, and the other using the
characteristic pattern of the large-scale structure as a cosmic ruler.
Negative pressure is believed to be a property of vacuum energy, but
the exact nature and existence of dark energy remains one of the great
mysteries of the Big Bang. Results from the
WMAP team in 2008 are in
accordance with a universe that consists of 73% dark energy, 23% dark
matter, 4.6% regular matter and less than 1% neutrinos. According
to theory, the energy density in matter decreases with the expansion
of the universe, but the dark energy density remains constant (or
nearly so) as the universe expands. Therefore, matter made up a larger
fraction of the total energy of the universe in the past than it does
today, but its fractional contribution will fall in the far future as
dark energy becomes even more dominant.
The dark energy component of the universe has been explained by
theorists using a variety of competing theories including Einstein's
cosmological constant but also extending to more exotic forms of
quintessence or other modified gravity schemes. A cosmological
constant problem, sometimes called the "most embarrassing problem in
physics", results from the apparent discrepancy between the measured
energy density of dark energy, and the one naively predicted from
Main article: Dark matter
Chart shows the proportion of different components of the universe
– about 95% is dark matter and dark energy.
During the 1970s and the 1980s, various observations showed that there
is not sufficient visible matter in the universe to account for the
apparent strength of gravitational forces within and between galaxies.
This led to the idea that up to 90% of the matter in the universe is
dark matter that does not emit light or interact with normal baryonic
matter. In addition, the assumption that the universe is mostly normal
matter led to predictions that were strongly inconsistent with
observations. In particular, the universe today is far more lumpy and
contains far less deuterium than can be accounted for without dark
matter. While dark matter has always been controversial, it is
inferred by various observations: the anisotropies in the CMB, galaxy
cluster velocity dispersions, large-scale structure distributions,
gravitational lensing studies, and X-ray measurements of galaxy
Indirect evidence for dark matter comes from its gravitational
influence on other matter, as no dark matter particles have been
observed in laboratories. Many particle physics candidates for dark
matter have been proposed, and several projects to detect them
directly are underway.
Additionally, there are outstanding problems associated with the
currently favored cold dark matter model which include the dwarf
galaxy problem and the cuspy halo problem. Alternative
theories have been proposed that do not require a large amount of
undetected matter, but instead modify the laws of gravity established
by Newton and Einstein; yet no alternative theory has been as
successful as the cold dark matter proposal in explaining all extant
The horizon problem results from the premise that information cannot
travel faster than light. In a universe of finite age this sets a
limit—the particle horizon—on the separation of any two regions of
space that are in causal contact. The observed isotropy of the
CMB is problematic in this regard: if the universe had been dominated
by radiation or matter at all times up to the epoch of last
scattering, the particle horizon at that time would correspond to
about 2 degrees on the sky. There would then be no mechanism to cause
wider regions to have the same temperature.:191–202
A resolution to this apparent inconsistency is offered by inflationary
theory in which a homogeneous and isotropic scalar energy field
dominates the universe at some very early period (before
baryogenesis). During inflation, the universe undergoes exponential
expansion, and the particle horizon expands much more rapidly than
previously assumed, so that regions presently on opposite sides of the
observable universe are well inside each other's particle horizon. The
observed isotropy of the CMB then follows from the fact that this
larger region was in causal contact before the beginning of
Heisenberg's uncertainty principle
Heisenberg's uncertainty principle predicts that during the
inflationary phase there would be quantum thermal fluctuations, which
would be magnified to cosmic scale. These fluctuations serve as the
seeds of all current structure in the universe.:207 Inflation
predicts that the primordial fluctuations are nearly scale invariant
and Gaussian, which has been accurately confirmed by measurements of
the CMB.:sec 6
If inflation occurred, exponential expansion would push large regions
of space well beyond our observable horizon.:180–186
A related issue to the classic horizon problem arises because in most
standard cosmological inflation models, inflation ceases well before
electroweak symmetry breaking occurs, so inflation should not be able
to prevent large-scale discontinuities in the electroweak vacuum since
distant parts of the observable universe were causally separate when
the electroweak epoch ended.
The magnetic monopole objection was raised in the late 1970s. Grand
unified theories predicted topological defects in space that would
manifest as magnetic monopoles. These objects would be produced
efficiently in the hot early universe, resulting in a density much
higher than is consistent with observations, given that no monopoles
have been found. This problem is also resolved by cosmic inflation,
which removes all point defects from the observable universe, in the
same way that it drives the geometry to flatness.
The overall geometry of the universe is determined by whether the
Omega cosmological parameter is less than, equal to or greater than 1.
Shown from top to bottom are a closed universe with positive
curvature, a hyperbolic universe with negative curvature and a flat
universe with zero curvature.
The flatness problem (also known as the oldness problem) is an
observational problem associated with a
Friedmann–Lemaître–Robertson–Walker metric (FLRW). The
universe may have positive, negative, or zero spatial curvature
depending on its total energy density.
Curvature is negative, if its
density is less than the critical density; positive, if greater; and
zero at the critical density, in which case space is said to be flat.
The problem is that any small departure from the critical density
grows with time, and yet the universe today remains very close to
flat.[notes 4] Given that a natural timescale for departure from
flatness might be the Planck time, 10−43 seconds, the fact that
the universe has reached neither a heat death nor a
Big Crunch after
billions of years requires an explanation. For instance, even at the
relatively late age of a few minutes (the time of nucleosynthesis),
the density of the universe must have been within one part in 1014 of
its critical value, or it would not exist as it does today.
Main article: Problem of why there is anything at all
Gottfried Wilhelm Leibniz
Gottfried Wilhelm Leibniz wrote: "Why is there something rather than
nothing? The sufficient reason [...] is found in a substance which
[...] is a necessary being bearing the reason for its existence within
itself." Philosopher of physics Dean Rickles has argued that
numbers and mathematics (or their underlying laws) may necessarily
exist. Physics may conclude that time did not exist before
'Big Bang', but 'started' with the
Big Bang and hence there might be
no 'beginning', 'before' or potentially 'cause' and instead always
existed. Some also argue that nothing cannot exist or that
non-existence might never have been an option.
Quantum fluctuations, or other laws of physics that may have existed
at the start of the
Big Bang could then create the conditions for
matter to occur.
Ultimate fate of the universe
Main article: Ultimate fate of the universe
Before observations of dark energy, cosmologists considered two
scenarios for the future of the universe. If the mass density of the
universe were greater than the critical density, then the universe
would reach a maximum size and then begin to collapse. It would become
denser and hotter again, ending with a state similar to that in which
it started—a Big Crunch.
Alternatively, if the density in the universe were equal to or below
the critical density, the expansion would slow down but never stop.
Star formation would cease with the consumption of interstellar gas in
each galaxy; stars would burn out, leaving white dwarfs, neutron
stars, and black holes. Very gradually, collisions between these would
result in mass accumulating into larger and larger black holes. The
average temperature of the universe would asymptotically approach
absolute zero—a Big Freeze. Moreover, if the proton were
unstable, then baryonic matter would disappear, leaving only radiation
and black holes. Eventually, black holes would evaporate by emitting
Hawking radiation. The entropy of the universe would increase to the
point where no organized form of energy could be extracted from it, a
scenario known as heat death.:sec VI.D
Modern observations of accelerating expansion imply that more and more
of the currently visible universe will pass beyond our event horizon
and out of contact with us. The eventual result is not known. The
ΛCDM model of the universe contains dark energy in the form of a
cosmological constant. This theory suggests that only gravitationally
bound systems, such as galaxies, will remain together, and they too
will be subject to heat death as the universe expands and cools. Other
explanations of dark energy, called phantom energy theories, suggest
that ultimately galaxy clusters, stars, planets, atoms, nuclei, and
matter itself will be torn apart by the ever-increasing expansion in a
so-called Big Rip.
The following is a partial list of the popular misconceptions about
Big Bang model:
Big Bang as the origin of the universe: One of the common
misconceptions about the
Big Bang model is the belief that it was the
origin of the universe. However, the
Big Bang model does not comment
about how the universe came into being. Current conception of the Big
Bang model assumes the existence of energy, time, and space, and does
not comment about their origin or the cause of the dense and high
temperature initial state of the universe.
Big Bang was "small": It is misleading to visualize the Big Bang
by comparing its size to everyday objects. When the size of the
Big Bang is described, it refers to the size of the
observable universe, and not the entire universe.
Hubble's law violates the special theory of relativity: Hubble's law
predicts that galaxies that are beyond Hubble Distance recede faster
than the speed of light. However, special relativity does not apply
beyond motion through space.
Hubble's law describes velocity that
results from expansion of space, rather than through space.
Doppler redshift vs cosmological red-shift: Astronomers often
refer to the cosmological red-shift as a normal Doppler
shift, which is a misconception. Although similar, the
cosmological red-shift is not identical to the Doppler redshift. The
Doppler redshift is based on special relativity, which does not
consider the expansion of space. On the contrary, the cosmological
red-shift is based on general relativity, in which the expansion of
space is considered. Although they may appear identical for nearby
galaxies, it may cause confusion if the behavior of distant galaxies
is understood through the Doppler redshift.
Main article: Cosmogony
Big Bang model is well established in cosmology, it is
likely to be refined. The
Big Bang theory, built upon the equations of
classical general relativity, indicates a singularity at the origin of
cosmic time; this infinite energy density is regarded as impossible in
physics. Still, it is known that the equations are not applicable
before the time when the universe cooled down to the Planck
temperature, and this conclusion depends on various assumptions, of
which some could never be experimentally verified. (Also see Planck
One proposed refinement to avoid this would-be singularity is to
develop a correct treatment of quantum gravity.
It is not known what could have preceded the hot dense state of the
early universe or how and why it originated, though speculation
abounds in the field of cosmogony.
Some proposals, each of which entails untested hypotheses, are:
Models including the Hartle–Hawking no-boundary condition, in which
the whole of space-time is finite; the
Big Bang does represent the
limit of time but without any singularity.
Big Bang lattice model, states that the universe at the moment of the
Big Bang consists of an infinite lattice of fermions, which is smeared
over the fundamental domain so it has rotational, translational and
gauge symmetry. The symmetry is the largest symmetry possible and
hence the lowest entropy of any state.
Brane cosmology models, in which inflation is due to the movement of
branes in string theory; the pre-
Big Bang model; the ekpyrotic model,
in which the
Big Bang is the result of a collision between branes; and
the cyclic model, a variant of the ekpyrotic model in which collisions
occur periodically. In the latter model the
Big Bang was preceded by a
Big Crunch and the universe cycles from one process to the
Eternal inflation, in which universal inflation ends locally here and
there in a random fashion, each end-point leading to a bubble
universe, expanding from its own big bang.
Proposals in the last two categories, see the
Big Bang as an event in
either a much larger and older universe or in a multiverse.
Religious and philosophical interpretations
Main article: Religious interpretations of the
Big Bang theory
As a description of the origin of the universe, the
Big Bang has
significant bearing on religion and philosophy. As a result,
it has become one of the liveliest areas in the discourse between
science and religion. Some believe the
Big Bang implies a
creator, and some see its mention in their holy books,
while others argue that
Big Bang cosmology makes the notion of a
Eureka: A Prose Poem, Edgar Allan Poe's
Big Bang speculation
Shape of the universe
^ There is no consensus about how long the
Big Bang phase lasted. For
some writers, this denotes only the initial singularity, for others
the whole history of the universe. Usually, at least the first few
minutes (during which helium is synthesized) are said to occur "during
the Big Bang".
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